The invention relates to improvements in hydrokinetic torque converters, especially for use in the power trains of motor vehicles, and to improvements in methods of operating such torque converters.
A hydrokinetic torque converter which is utilized in the power train of a motor vehicle forms part of an operative connection between the prime mover (such as an internal combustion engine or a hybrid drive) and the change-speed transmission. Reference may be had, for example, to commonly owned U.S. Pat. No. 5,782,327 granted Jul. 21, 1998 to Otto et al. for “HYDROKINETIC TORQUE CONVERTER AND LOCKUP CLUTCH THEREFOR”, as well as to the publications which are discussed in the specification of this patent. The efficiency of the torque converter can be enhanced by resorting to a so-called bypass or lockup clutch which, as a rule, becomes engaged when the ratio of the RPM of the turbine to the RPM of the pump in the housing of the torque converter is between about 0.8 and 0.85. The efficiency of the torque converter rises to or approximates 100% as soon as the bypass clutch is engaged. Proper operation of the torque converter is ensured if its housing is at least substantially filled with a suitable hydraulic fluid (such as oil) at all times. Furthermore, and since the hydraulic fluid is likely to be heated and contaminated, it is desirable to ensure continuous circulation and simultaneous cooling and/or cleaning of such fluid.
Certain torque converters are designed in such a way that the fluid stream serves to operate and lubricate the bypass clutch. The arrangement can be such that the stream of fluid which issues between the turbine and the pump of the torque converter bears upon an axially movable disc which forms part of the bypass clutch, which is concentric with the turbine and which is provided with a friction lining. When the pressure of the fluid stream increases, the disc is caused to urge its friction lining against the adjacent portion of the internal surface of the rotating torque converter housing, i.e., the bypass clutch is engaged and transmits torque from the housing (which is driven by the rotary output element of the prime mover) to the rotary input element (e.g., a shaft) of the change-speed transmission. The bypass clutch can become disengaged when the pressure of the fluid stream decreases or when the direction of fluid flow is reversed, depending upon the design of the torque converter and of its bypass clutch.
It is also known to design a torque converter for use in the power train of a motor vehicle in such a way that its bypass clutch is engaged in response to admission of hydraulic fluid into a so-called third channel or passage. If the pressure of fluid in the third channel decreases, the circulating fluid stream (which is supplied by a first and a second channel) disengages the bypass clutch. A torque converter which is provided with a third channel normally employs a bypass clutch having a package of several laminations or discs with friction linings thereon, i.e., the engagement of such bypass clutch involves the transmission of torque by the entire package of such laminations.
In order to avoid or reduce fluctuations of torque in the power train of a motor vehicle, the torque converter in the power train can be operated in such a way that it becomes disengaged in a certain manner when subjected to load. Thus, the bypass clutch is caused to generate (at least in a certain portion of the power train) oscillations which oppose normal fluctuations of torque in the power train, i.e., the normal fluctuations of torque become less pronounced with attendant reduction of their adverse influence upon the power train.
That stage of operation of a clutch when the surfaces of linings, laminations and analogous torque transmitting parts are in sliding contact with each other and/or with the friction generating parts is known as operation with slip. Such mode of operation invariably entails the generation of heat irrespective of the nature of the substances (such as metal, special friction lining material or the like) of which the laminations are made or which are utilized as coatings for metallic or other laminations. The thus heated discs exchange heat with the hydraulic fluid in the housing of the torque converter. Overheating of the discs entails damage thereto, especially if the discs are not lubricated in a required manner. Furthermore, overheating can result in at least partial decomposition of the hydraulic fluid. The thus decomposed hydraulic fluid and/or broken-off particles of metallic laminations, friction linings and torque converter housing can affect the operation and hence the reliability of the torque converter and/or its bypass clutch and/or of the power train.